Method and Arrangement for Improved Outer Loop Power Control

ABSTRACT

A method for improved outer loop power control in a communication system, comprising collecting (S 1 ) quality measurements for a plurality of sub-block within a received coding block; mapping (S 2 ′) the collected sub-block level quality measurements to a block level quality measure, and adjusting (S 3 ′) the inner loop power control requirement based at least on a representation of the block level quality measure.

This application is the U.S. national phase of International ApplicationNo. PCT/SE2004/001989 filed 22 Dec. 2004 which designated the U.S., theentire content of each of which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to communication systems in general,specifically to improved power control in interference limitedcommunication systems.

BACKGROUND

The Transmit Power Control (PC or TPC) functionality is crucial forinterference limited cellular systems, such as WCDMA, GSM and cdma2000,where more high-powered mobiles interfere with weaker mobiles in thesystem. Power control adjusts the transmit power from a transmitter,e.g. mobile station, in order to maintain a certain quality at areceiver, e.g. a base station. This reduces the interference caused bythe transmitter to other receivers. Also, the battery power for themobile stations is conserved.

In systems utilizing Code Division Multiple Access (CDMA), known powercontrol functionalities that operate on the Signal-to-Interference-Ratio(SIR) include Outer Loop Power Control (OLPC) and INner loop PowerControl (INPC). The OLPC is responsible for compensating for channel orlink variations by adjusting the SIR target for the INPC which issensitive to the accuracy of instantaneous quality measurements. TheOLPC is typically based on the block error indicator (BEI) checked by aCyclic Redundancy Check (CRC),

The INPC subsequently compares the estimated SIR at the receiver withthe SIR target and adjusts the transmitted power accordingly. If theestimated SIR is higher than the SIR target, a Transmit Power Control(TPC) command to decrease the transmit power is signaled to thetransmitter and vice versa if the estimated SIR exceeds the SIR target.

The problem with OLPC, according to prior art, is that it is notsuitable to use in cases where the BLER is an inaccurate measure of thequality. Also, the current OLPC has in many cases very slow convergence

Some attempts in prior art to provide improved outer loop power controlhas included adding a middle loop between the outer and the inner loop[2], [3] rather than actually improving the OLPC.

Therefore, there is a need for an improved OLPC which is faster andprovides a better estimate of end user quality.

SUMMARY

An object of the present invention is to improve outer loop powercontrol.

A specific object is to enable outer loop power control based on qualitymeasurements other than BLER.

Another specific object is to enable outer loop power control forservices with adaptation of the source coding rate.

Another specific object is to enable outer loop power control based onsub-block quality measurements.

Yet another specific object is to enable determination of an inner looppower control requirement based on sub-block quality measurements.

These and other objects are achieved in accordance with the attachedclaims.

Briefly, the present invention comprises collecting sub-block levelquality measurements and adapting an inner loop requirement based atleast on those measurements.

More specifically, the invention comprises collecting sub-block levelquality measurements and mapping those to a block-level quality measure,and subsequently adapting the inner loop power control requirement basedat least on a representation of the block-level quality measure.

Some of the advantages with the present invention are:

-   -   An outer loop power control that operates on quality rather than        BLER    -   A quality model that can be updated on-line    -   An outer loop power control with fast convergence    -   Accurate quality estimate after one received block

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of power control in a communicationsystem;

FIG. 2 is a schematic illustration of source coding rate adaptation;

FIG. 3 is a schematic flow diagram of an embodiment of a methodaccording to the invention;

FIG. 4 is a schematic block diagram of an embodiment of an arrangementaccording to the invention;

FIG. 5 is a schematic block diagram of an embodiment of an arrangementaccording to the invention.

DETAILED DESCRIPTION

Frequently mentioned abbreviations are listed below:

-   -   AMR Adaptive Multi-Rate. New speech coder used in GSM [1]    -   BER Bit Error-Rate. The number of erroneous bits divided by the        total number of bits. The term rawBER means the BER at the input        of the channel decoder.    -   BLEP BLock Error Probability. The probability that a single        block is incorrect    -   BLER Block Error-Rate. The number of erroneous blocks divided by        the total number of blocks.    -   EMR Enhanced Measurement Report. A new type of reporting scheme,        where, the mobile reports the mean and standard deviation of the        rawBER once every 480 ms. [4]    -   FER Frame Erasure Rate. The fraction of speech frames that were        erased, i.e., discarded due to transmission errors.    -   SQI Speech Quality Index. An attempt to estimate the speech        quality based on objective measurements, e.g. BER and BLER. [5]

In order to provide a clear and thorough understanding of the presentinvention, the problems with prior art will be discussed in detailbelow.

According to prior art, the power control is typically closed loop,which means that the system actually performs measurements that areaffected by the power that the transmitting link uses.

With closed loop power control, the receiver measures some quantity ofthe received signal. It is important that this quantity is easy tomeasure, and that the measurement period is short, so that the transmitpower can be frequently updated. The most common example of such ameasurement is the Signal-to-Interference Ratio (SIR). This quantity maybe measured very frequently. In WCDMA, SIR is measured 1500times/second. The transmit power is adjusted so that the measured SIR isclose to the so-called SIR target: if the measured SIR is higher thanthe SIR target, the transmit power is reduced, whereas the transmitpower is increased if the measured SIR is lower than the target. In astationary environment, this will force the SIR estimate to be equal tothe SIR target.

However, although the power control forces the SIR measurement to beequal to the SIR target, the received quality may not be sufficient.First, the SIR estimate may be inaccurate. Second, different receiversmay require different SIR to achieve the same received quality, i.e. SIRmay not be a very good quality measure. To overcome these twolimitations, OLPC has been introduced in prior art, to complement theINPC. This is schematically illustrated in FIG. 1. The OLPC adjusts thetarget of the INPC to compensate for e.g. the two shortcomings mentionedabove. The OLPC should operate on a quantity that is closely related tothe actual received quality.

For the majority of services in interference limited communicationsystems, the service quality is closely related to the BLER i.e. thenumber of erroneous blocks divided by the total number of receivedblocks. Therefore, the OLPC, according to prior art, typically operateson the BLER, and each service is associated with a certain BLER target.In principle, the OLPC functionality increases the SIR target if theBLER exceeds the BLER target, and decreases the SIR target if the BLERis lower than the BLER target.

One problem with OLPC based on BLER, according to prior art, is that forsome services the service quality is not directly related to the BLER.One important example of such a service is the so-called AdaptiveMulti-Rate (AMR) [1] speech coder, recently introduced in commercial GSMsystems.

With AMR, the source and channel-coding rate is adapted to the channelquality. The speech-coding rate changes from 4.75 kbps with the MR475mode to 12.2 kbps with the MR122 mode. When the channel quality is bad,a strong channel coder that adds a lot of redundancy is used, and whenthe channel quality is good, a weaker channel coder that adds only asmall amount of redundancy is used. With the strong channel coding,successful transmission of the information bits becomes possible atworse channel conditions. Since the number of bits transmitted over theradio interface is fixed, the speech coder must adapt to deliver fewerbits. This principle is shown in FIG. 2.

Providing that all bits are received correctly, the speech quality isbetter the higher the speech-coding rate. However, the speech qualitydeteriorates when there are errors in the transmission. Therefore, aspeech coder with lower rate where all the bits are received correctlymay provide better speech quality than a high-rate speech coder withtransmission errors. This is the reasoning behind AMR: it is better to“spend” the available bits on error protection, i.e. channel coding, ifthe channel is bad, than on more accurate speech coding i.e. a highspeech coding rate.

A direct consequence of the adaptation of the speech coding rate is thatthe speech quality is not directly connected to the BLER anymore: thespeech quality also depends on what speech coder was used for thetransmission. In this case, a combination of the speech coding rate andBLER would provide a measure of the speech quality. Consequently, OLPCaccording to prior art cannot be used.

Even with fixed-rate speech coders, BLER may not be a good qualityestimate. For instance, the speech quality is also affected by thedistribution of block errors, an effect that has been included in thecalculation of SQI [5]. In principle, it should be possible to design anOLPC that operates on SQI rather than directly on BLER.

Another problem with BLER is that for those BLER levels that areinteresting in a cellular scenario (less than or equal to 1%) it takes along time to get a good estimate of the BLER: the number of blocksrequired is in the order of 1/BLER.

In light of the above mentioned limitations, there is a need forimproved power control, specifically improved outer loop power control,which can operate in situations where BLER is an inaccurate measure ofquality, due to e.g. variations in the source coding rate.

The present invention overcomes these problems by providing an indirectestimate of a quantity that provides a measure of the service quality.

Digital information is normally transmitted in blocks. In this context,a block is the smallest unit that has a checksum attached to it tofacilitate error detection. The information in a block is typicallysplit into several sub-blocks before it is transmitted over the radiointerface. In WCDMA, the sub-blocks are called slots, and in GSM theyare called bursts.

A basic embodiment, with reference to FIG. 3, of a method according tothe invention comprises collecting S1 some quality measure, e.g. SIR,rawBER or mutual information of the received sub-blocks into which theblock has been split. The concept of mutual information is described in[6]. Subsequently, the inner loop power control target or requirement isadjusted S2 based at least on the sub-block level quality measure or arepresentation thereof.

According to a more detailed embodiment, the collected sub-block levelquality measure is mapped S2′ to a block level quality measure, e.g.SQI, BLEP, for each block. Subsequently, the inner loop power controltarget is adjusted S3′ based at least on the block level quality measureor a representation thereof.

Some exemplary mappings are discussed in [6]. Two examples of suchmappings are given below:

1. In GSM, the rawBER can be estimated for every sub-block i.e. burst. Aspeech block is transmitted over eight consecutive bursts. Over thesebursts, the mean and standard deviation of the rawBER can be calculated.With the introduction of the Enhanced Measurement Report (EMR) [4],these rawBER quantities become available both for uplink and downlink.Subsequently, the mean and the standard deviation of the rawBER aremapped to BLER or FER. Consequently, a two dimensional mapping is usedfor GSM i.e. two input values generate one output value. This model isdescribed in some detail in [7].

2. In WCDMA, the SIR can be collected over the sub-blocks i.e. slotsthat are used to transmit a block. It has been shown that an exponentialaverage of the SIR values form a good estimate of the link quality:

${SIR}_{eff} = {\frac{1}{C}{\ln( \frac{\sum\limits_{m = 1}^{N}{\exp ( {{- C}\; \eta_{m}} )}}{N} )}}$

where ln is the natural logarithm, η_(m) is the SIR of slot m and N isthe number of sub-blocks within a coding block, and C is a servicedependent constant that can be empirically determined with relativeease. SIR_(eff) can then either be used directly as a quality measure ormapped to BLER using data from a simulation.

The output of these models can be thought of as Block Level ErrorProbability (BLEP), which is an estimate of “instantaneous BLER”. TheOLPC may then use BLEP instead of BLER. This means that already afterone received block, the receiver will be able to estimate BLER withrelatively good accuracy. Also, with AMR, it is possible to get anestimate of the BLEP that would be experienced if another speech codingmode were used, i.e. it is possible to estimate the quality of MR475although MR122 is actually used, simply by using the right qualitymodel.

The quality model can, according to one embodiment, be obtained by alink simulation, where the sub-block level quality measure is collectedand grouped according to the chosen mapping, e.g. all blocks where theaverage and standard deviation of the rawBER falls into a specificinterval are collected and the BLEP estimated from the fraction oferroneous blocks.

The quality model can, according to one embodiment, be updated on-lineby using the same procedure: the sub-block-level quality measure iscollected and mapped to the block-level quality measure. The status ofthe block (block error or not) is then checked and the result is used toupdate the quality model. Again taking the GSM mapping 1 as an example,for all received blocks with average rawBER and standard deviation ofrawBER inside a predetermined interval, the total number of blocks andthe number of erroneous blocks are counted. The quotient is then used asan estimate of the BLEP. If desired, different models can beautomatically tuned for different terminal types. This would make sensefor instance if the accuracy of the estimate of the sub-block levelquality differs among different terminal types.

An embodiment of an arrangement according to the invention will bedescribed below with reference to FIG. 4.

The arrangement 10 includes an I/O unit for receiving input signals andproviding output signals to and from the arrangement, a unit formeasuring and collecting quality measurements 11, a unit for determiningan inner loop power control requirement or target 12.

The arrangement will be referred to as but not limited to an OLPC unit.The arrangement can equally be incorporated for providing an inner looppower control target in a more general power control arrangement.

According to the embodiment in FIG. 4, the quality measuring unit 11 isadapted to collect and/or measure some quality parameter or measurementfor a plurality of sub-blocks within a received coding block. Thequality measurement can comprise any one of rawBER, SIR, or mutualinformation or some other quantity.

The determining unit 12, according to the embodiment, is adapted todetermine or adjust the inner loop power control target based on atleast the sub-block level measurements or a representation thereof.

According to another specific embodiment, the arrangement 10 comprises amapping unit 13 which is adapted to map the collected sub-block levelquality measures to a corresponding block level quality measure. This ispreferably performed by utilizing a predetermined quality model, asdescribed previously. The determining unit 12 is then adapted to adjustor determine the inner loop power control requirement based on at leastthe block-level quality measurement or a representation thereof.

According to yet another specific embodiment, the mapping unit 13 isadapted to update the quality model based on which type of terminal isreceiving the coding blocks, the type of source coder, etc.

With reference to FIG. 5, an embodiment of a power control unit 20,according to the invention, comprises an inner loop power control unitINPC and an OLPC unit 10, in which said OLPC unit comprises a qualitymeasurement unit 11, a target determining unit 12 and an optionalmapping unit 13. The functionality of each unit is as describedpreviously.

Also, with reference to FIG. 5, an embodiment of a mobile station 30,according to the invention, comprises a power control unit 20, accordingto the above embodiment.

Basically, the invention enables the OLPC to operate on an estimate ofthe instantaneous quality, which is estimated using sub-block levelmeasurements. Thus, the invention makes it possible for the OLPC toadjust the INPC target based on a quality estimate other than BLER. Thequality estimate could be the BLEP of the transmitted block. It couldalso be the BLEP that would be expected if another block weretransmitted. It can also be some other quantity, such as SQI.

Some of the advantages of the present invention include:

-   -   OLPC with increased convergence rate    -   OLPC based on quality rather than BLER    -   Quality model which can be updated on-line.    -   Accurate quality estimate already after one received block.

It will be understood by those skilled in the art that variousmodifications and charges may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

REFERENCES

-   [1] 3GPP, “Performance Characterization of the GSM Adaptive    Multi-Rate (AMR) speech coder”, GSM 06.75, version 7.2.0, Release    1998-   [2] “Power control in a CDMA mobile communication system”,    WO01/20808 A3-   [3] “Power control in a mobile radio communication system”,    WO03/055098 A1-   [4] 3GPP, “Radio subsystem link control”, TS 45.008, version 5.14.0,    release 5-   [5] Stefan Wanstedt et al., “Development of an objective speech    quality measurement model for the AMR codec”, On-line workshop    MESAQIN 2002.-   [6] 3GPP2, “Effective-SNR Mapping for Modeling Frame Error Rates in    Multiple-state Channels”, 3GPP2-C30-20030415, source Ericsson-   [7] Håkan Olofsson et al., “Improved interface between Link Level    and System Level Simulations Applied to GSM”, ICOPC'97, 1997

1. A method for improved outer loop power control in a communicationsystem, characterized by: collecting quality measurements for aplurality of sub-block within a received coding block; and adjusting aninner loop power control requirement based at least on the collectedsub-block level quality measurements.
 2. The method according to claim2, characterized by the further steps of: collecting qualitymeasurements for each of the plurality of sub-blocks within the receivedcoding block; mapping the collected sub-block level quality measurementsto a block level quality measure; adjusting the inner loop power controlrequirement based at least on a representation of the block levelquality measure.
 3. The method according to claim 1, characterized bysaid sub-block level quality measurements comprises estimates of one of,rawBER, SIR, or mutual information.
 4. The method according to claim 2,characterized by mapping said sub-block level quality measurement to oneof BLEP or SQI.
 5. The method according to claim 2, characterized bymapping said sub-block level quality to said block-level qualityutilizing a predetermined quality model.
 6. The method according toclaim 5, characterized by updating said quality model based onsub-block-level measurements over several received blocks.
 7. The methodaccording to claim 6, characterized by updating said quality model basedon which type of terminal is receiving the blocks.
 8. The methodaccording to claim 6, characterized by updating said quality model basedon which source-coding rate is used for the received blocks.
 9. A devicefor outer loop power control in a communication system, characterizedby: means for collecting quality measurements for a plurality ofsub-blocks within a received coding block; and means for adjusting aninner loop power control requirement based at least on the collectedsub-block level quality measurements.
 10. The device according to claim9, characterized by said device further comprising: means for mappingthe collected sub-block level quality measurements to a block levelquality measure; and said adjusting means are adapted for adjusting theinner loop power control requirement based at least on a representationof the block level quality measure.
 11. The device according to claim 9,characterized in that said collecting means are adapted for collectingsub-block level quality measurements comprising estimates of one ofrawBER, SIR, or mutual information.
 12. The device according to claim10, characterized in that said mapping means are adapted for mappingsaid sub-block level quality to said block-level quality utilizing apredetermined quality model.
 13. The device according to claim 10,characterized in that said block level quality measure comprises one ofBLEP or SQI.
 14. The device according to claim 12, characterized in thatsaid mapping means are adapted for updating the predetermined qualitymodel based on sub-block-level measurements over several receivedblocks.
 15. The device according to claim 12, characterized in that saidmapping means are adapted for updating the predetermined quality modelbased on the type of terminal that is receiving the blocks.
 16. Thedevice according to claim 12, characterized in that said mapping meansare adapted for updating the predetermined quality model based on whichsource-coding rate is used for the received blocks.
 17. A power controlunit in a communication system, characterized by a device for outer looppower control according to claim
 9. 18. A mobile terminal, characterizedby a power control unit according to claim 17.